1. Field of the Invention
The present invention relates to a calibration device for radiography and X-ray computed tomography systems, and more specifically, to a calibration phantom which incorporates calcium into a human tissue equivalent material in terms of x-ray beam attenuation and scatter, and methods of fabrication and use of the same.
2. Description of the Related Art
Osteoporosis is the most common disorder of the human skeletal system, affecting up to 32 percent of women and 17 percent of men, depending upon the age group under consideration, Basically, osteoporosis is a disease process in which the mineral content (i.e., calcium content) of a person's skeletal system is gradually reduced, leading to a higher risk of fractures particularly in the spine, hip, and wrist. Osteoporosis is a major medical problem. It has been estimated that approximately 40,000 American women die per year from complications due to osteoporosis.
In the past, osteoporosis was considered to be undiagnosable prior to the onset of symptoms, and untreatable once it became symptomatic. Thus, it was frequently called the "silent disease." More recently, however, techniques have been developed which detect the early mineral loss in a person's bones. Such techniques include computed tomography (CT) quantitative computed tomography (QCT) and dual-energy x-ray absorptometry (DEXA).
Computed tomography uses an array of detectors to collect x-ray attenuation data from x-ray beams that pass through the body. The data are input as digital data to a computer, which processes that data and reconstructs planar cross-sectional images of the internal structures of the body through which the x-ray beams pass.
DEXA uses a dual energy approach to compensate for tissue variations to allow quantification of bone mass in a projection image. QCT requires the use of a bone-equivalent calibration phantom which is scanned simultaneously with the patient to provide bone density measurements in axial images.
Each of these methods require access to and use of sophisticated and relatively expensive equipment. In addition, the images produced can vary significantly in response to a number of technical factors related to the apparatus used, as well as errors caused by beam hardening and scattered radiation within the human body.
Plain film radiographs are frequently taken to qualitatively assess bone density throughout the body. Conventional radiographic apparatuses are widely available throughout the world and thus allow easy access for most patients. Although these radiographs provide very high spatial resolution and indicate relative attenuation of neighboring tissues, they are highly inaccurate and subject to gross misjudgment in assessing the patient's condition in terms of bone mass. Due to a variety of technical factors, quantification of calcium density from single energy projection radiography has not been possible.
In particular, quantitative x-ray measurements are influenced by x-ray beam hardening due to the broad spectral distribution of x-rays. As the x-rays pass through tissue or any other medium, lower energy x-rays are preferentially absorbed. This results in a shifting of the effective beam energy to higher values. Thus, the quantitative results which are obtained will vary with the size and shape and composition of the particular patient's anatomy.
The detection and quantification of calcification in pulmonary nodules, coronary arteries, aortic calcification, breast tumors and the like has been a goal of clinical radiology for some time. Cine CT, dual energy digital subtraction fluoroscopy, and dual energy film subtraction radiography have been tried. It has long been desirable, however, to quantify calcium or bone density in conventional x-ray projection images without using dual energy techniques.
Stepwedges using material of varying thickness are frequently used in radiology for quality control testing of x-ray beam properties. By varying the thickness of the steps, the intensity and spectral content of the x-ray beam in the projection image can be varied.
Stepwedges are commonly made of aluminum, copper and other convenient and homogeneous materials of known x-ray attenuation properties. Stepwedges using bone-like absorption materials have been used in quality control tests to evaluate the ability of dual energy imaging to quantify pulmonary nodules, see Kruger, et al., "Dual Energy Film Subtraction Technique for Detecting Calcification in Solitary Pulmonary Nodules," Radiology, Vol. 140, pages 213-219, July 1981. Previous efforts have used bone phantoms imaged separately from the patient to test the sensitivity of the technique for quantification. These stepwedge-like phantoms use calcium phosphate powder or calcium phosphate powder in molten paraffin. Since the phantoms use powder and/or paraffin, they lack packing consistence, long term stability, and the homogeneity of mixing, which are desirable characteristics of a phantom used repeatedly over long periods of time.
There is therefore a substantial need for an improved test phantom representative of human tissue containing calcium in a long-term stable format, and a low cost method of quantifying bone density and calcium content which is fast, accurate, reproducible and widely available.